Global Positioning System or GPS is a satellite-navigation system that consists of a network of 24 satellites placed into orbit by the U.S. Department of Defense. Originally GPS was solely intended for military use, costing the U.S. billions of dollars to build and maintain. Finally in the 1980's the government made the system available for civilian use. GPS was used by hikers to navigate through the wilderness and by boaters and fishermen to find their whereabouts in the open sea and surveyors use GPS to position themselves on a particular site within one centimeter of their intended destination. The most popular GPS application and the most recent is the navigation system within a motor vehicle.

A Brief History of Navigation

An astrolabe. This device is a predecessor of the sextant, a device that defined early surveying.

Throughout history, exploration has always been a vital activity for man. Exploration leads to discoveries and discoveries lead to riches. The rate at which man has always explored is a product of the navigational vices available at the time. More than a millennium ago explorers were using bodies of water in combination with stars and the sun to assist in determining their position and direction relative to their start location. While maps did exist during the aforementioned times, the integrity of each individual map was at the hands of the explorer and/or cartographer that was involved in the construction of it. The fact that the sky was not always clear also brought upon troubles for the early explorer.

It was not until the beginning of the 12th century A.D. that the Chinese were able to apply a new device known as that magnetic compass to exploration and enable themselves to continue on to an intended destination even when the weather was overcast. Such an innovation greatly helped the sailors of the time. Other point location devices, such as the Cross-staff, existed during this time but sailing applications were not realized until early 15th century.

Near the mid 1600s, a man by the name of Pierre Vernier reinvented the Davis Quadrant and refined it in such a way that allowed a location to be determined within one nautical mile of a sailors actual location. In the follow century devices known as the Octant and the Sextant were developed to help “sight” objects using both horizontal and vertical angles. The increased precision of the sextant meant that it would easily take over its predecessors as the most reliable instrument. In fact, it is still a viable option for use on ships today if the connection with GPS satellites were to fail.

Radios were beginning to be used early in the 20th century. Through the use of telegraphs, people on land could warn incoming sailors after eminent dangers near shore. Light houses were soon developed for use in combination with radio telegraphs to direct ships into proper channels and ports to further reduce the chances of running into unseen objects. In the early 1960s, the Soviet Union launched a satellite by the name of sputnik. Within five years, a new system named TRANSIT was launched and working with a total of seven satellites and an expect accuracy within 80 feet.

This is a sextant. Developed after the octant, improved precision, accuracy, and the ability to effectively measure longitude.

How GPS Works

For the GPS (in this case Earthmate PN-20) to know where it is located, the GPS needs to know where the satellites location and distance as compared to the GPS. The GPS receives two different types of data the almanac and ephemeris. The almanac data contains approximate positions (locations) of the satellites, which is continuously transmitted and stored in the memory of the GPS receiver so it knows where the orbits of the satellites and where each satellite is suppose to be. If a satellite travels slightly out of orbit, ground monitoring stations are always keeping track of the satellite orbits, altitude, location and speed. The ground monitoring stations are always sending data to a master control station, which in turn sends corrected data up to the satellites, this is corrected data is known as ephemeris. Once the GPS receiver knows the location of a satellite, the receiver must determine the distance between the receiver and the satellite so that it can determine its position on earth. The velocity of the radio wave is 186,000 miles per second (speed of light) and the time it takes for the signal to get to the receiver determines the distance between the receiver and the satellite, thus giving the GPS receiver a location.

There are many sources of error associated with GPS. Signals can be slowed as it passes through the atmosphere, the system uses a built-in model that calculates an average, but no exact amount of delay can be determined. Signal multi-path errors originate from signals reflecting off of tall buildings or large rock sources before it reaches the receiver. These same tall buildings can also block signals from satellites, the smaller amount of satellites the less accurate the GPS becomes.

W.A.A.S.

Wide Area Augmentation System or WAAS was first created by the Federal Aviation Administration (FAA) to provide exceptional positioning information when pilots are found in deteriorating weather conditions or when visual navigation becomes impossible. A network of 25 ground reference stations or Wide Area Reference Stations (WRS) that cover the entire U.S. and some of Canada and Mexico. Each station is in a precisely surveyed location where they compare GPS distance measurements to known values. Each station is connected to the WAAS Master Station (WMS) where GPS information is collected and forwarded to through a terrestrial communications network. At the WMS, the WAAS augmentation messages are generated these messages contain information that allows GPS receivers to remove errors in the GPS signal. The errors removed from the signal increase the accuracy of the measured location. In aircrafts WAAS receivers typically have accuracies of 3 to 5 meters horizontally and 3 to 7 meters in altitude.

WAAS is now being used by regular GPS receivers for a more precise location. WAAS also provides indications to GPS?WAAS receivers of where the GPS system is unusable due to system errors or other effects. Also the WAAS system was designed to notify users within six seconds of any issuance of hazardously misleading information that would cause an error in the GPS position estimate.

Sources of Error

Selective Availability

Atmospheric Distortion

According to the GPS the earth’s atmosphere is divisible into two layers, the ionosphere and the troposphere. The troposphere is comprised of stratosphere and the mesosphere and spans from the grounds surface up to 80 km. The ionosphere is the layer that extends from the top of the troposphere up to 1000 km above the earth’s surface. The ionosphere is highly populated with electrons and positive ions due to the solar and cosmic radiation stripping electrons and ionizing gases. The GPS signal travels at the speed of light before entering the atmospheric zones, both the troposphere and the ionosphere refract the signal and reduce the speed of propagation. The troposphere tends to alter the signal accuracy by 1m, whereas the ionosphere tends to have a more significant affect on accuracy. The ionosphere alters result accuracies depending on the electron density, time of day and elevation of the receiver above the horizon. It is presumed that on average the maximum error due to the ionosphere is around 10m (approximately 30ft). The reduced signal speed through the atmospheric zones results in elongated signal distances and skewed readings therefore all GPS receivers are equipped with a mathematical program to account for the ionosphere correction factor.

Relativistic effects

Noise, bias and blunder

Signal multi-path

The multipath effect is caused by the reflection of the GPS signal on nearby surfaces or objects such as tall buildings. It is known that large bodies of waters are the most reflective and therefore detrimental to the GPS accuracy. This effect occurs when the signal, produced by the satellite, reaches the receiver by more than one path due to an obstruction. The reflected signal then takes a longer amount of time to reach the receiver than the direct signal. This allots to an inaccuracy of anywhere between 0-20 meters but is highly within the 0-1 meter range.

Receiver clock errors

Ephemeris Errors

Improving Accuracy & DGPS

Uses of GPS

Global Positioning System, GPS, has a wide variety of uses for the military as well as everyday applications for the private sector.

Private Sector Uses

The most common use of GPS in the private sector is for navigation. Navigation is becoming a standard feature in most automobiles. The navigation units in these automobiles do more than take you from point A to point B, they also list numerous points of interest, helping people unfamiliar with an area find these locations. This GPS navigation is not only limited to automobiles, it is used in boats, as well as planes. On commercial, as well as recreational, boats, GPS is used to navigate around the waters. It provides a much more accurate navigation assistance than the manual navigation tools from years past. Airplanes, commercial and private, use GPS technologies as well. Air traffic control towers use the GPS technology to track every plane that is in the air. The information that is gathered can help prevent disasters, such as in air collisions. The pilots of the plane use GPS to help them stay on route, preventing such disasters as well.

Hand-held GPS units are being used for a wide variety of recreational activities. Snowmobiling, hiking, biking, hunting, etc., are some of the recreational activities that use GPS devices to help navigate through the woods, mountains, and other areas. It allows these people to venture off the trails without fear of getting lost.

The majority of cellular phones are now being equipped with GPS capabilities. These phones are able to give turn by turn directions. These GPS capabilities also allow that phones location to be pin-pointed. This can be done on a personal computer, but more importantly, it is used by 911 services to find out the location where emergency calls are placed.

The use of GPS is starting to be used in the construction field. GPS devices are being installed on earth moving equipment in order to make grading job sites easier. The device that is mounted on the machine works with earthwork design programs such as Agtek. The site file, with the correct grades, is loaded into the on board computer, and the program shows the operator what level to have the blade, in order to achieve the correct grades. This technology is eliminating the need for a surveyor to stake the site before grading can begin. GPS surveying equipment is also allowing great advances in the study of earthquakes.

Surveyors are using GPS technology to get basic information about a site that they are working. This information used to be gathered by using optical surveying devices as well as other measuring devices. A surveyor would start with a known point and then use these devices to find out information about other points on the site. In order to complete the gathering of information on a site, the surveyor and the crew would spend numerous days on the site, and only be able to gather the information during the daylight hours. With the use of GPS, the time needed to gather information in the site is reduced drastically. Since it is uses satellites to obtain the needed information, the surveyor could work into the night. The GPS unit works by using satellites to record the different signals that are received when the device is taken to the desired locations. All of the gathered information can be logged into the computer, and the necessary maps could be made.

Military Uses

Like the private sector, the military uses GPS for navigation purposes. It helps the soldiers coordinate movements for supplies and missions. It also gives them the ability to move at night or in an area that they don't have much information on. When gathering information on an area, reconnaissance missions are performed, these missions, coupled with GPS technologies allow maps to be created of the area to aid in war.

Soldiers and pilots are being equipped with GPS receivers, so that in the event that they are lost, or an aircraft goes down, it is easier for their fellow soldiers to find and rescue them.

GPS is used to track and identify possible enemy targets. These GPS units are able to track targets on the ground and in the air. Once the target is confirmed to be an enemy, it can be engaged by the many GPS guided weapons. Missiles, precision guided munitions, etc. Having the exact coordinates of the enemy, and the use of the GPS, minimizes civilian casualties.

Calculating Positions with GPS

(Alison Lupariello)

Satellites in 6 Orbital Planes

There are three main components of the GPS system and these are the satellites that transmit the position information, the ground stations that control these satellites, and the individual GPS receivers that collect the data from the satellites and calculate the exact locations. Before GPS systems existed, a person could determine their position on the earth by using three known locations and respective compass bearings. Drawing a line through each of these three locations would create a triangular intersection. It could be determined that the person's location was within this area, but the size of this triangle would display the imprecision or inaccuracy of the lines or sightings.

The GPS technology used today demonstrates similar concepts, however, it uses the distances found from satellite signals to create imaginary spheres that intersect at the receiver's location. The distance from the satellite to the GPS receiver is known as the pseudorange, and it is calculated by measuring time it takes for the signal to be transmitted from the satellite and reach the receiver. Then, assuming that the signal travels at the speed of light, the pseudorange can be calculated. Then, using the satellite location and pseudorange, an imaginary shere can be formed around the satellite, knowing that the receiver's location lies at a point on the outside of this sphere. These steps are repeated for two more satellites in order to form two additional spheres that will intersect to determine the exact location of the GPS receiver. A representation of these overlapping psedoranges is pictured below, and the small triangular intersection shows the very small range of imprecision in determining the exact position.

Curves of Overlapping Pseudoranges

Although it only requires three pseudoranges and "imaginary spheres" to determine the exact position, a fourth satellite is used to confirm this location. This last satellite should theoretically intersect with the other three at the exact same position, and each of these groups of four satellites is known as a "geometric dilution of precision (GDOP)". Any precision error in this intersection is able to calculated, corrected, and re-computed to determine an even more accurate location of the receiver. Due to the fact that these satellites and receivers are constantly moving, these calculations are being continuously repeated and "new orbital position data, or ephemeris" is being downloaded to calculate the updated satellite positions.

DeLorme Earthmate GPS PN-20

General Information

The unit being disassembled below is the DeLorme Earthmate GPS PN-20. It was received in new condition. The entire contents of the package are shown below:

This GPS unit is compatible with computers, cars, and boats. It can also be used as a handheld device with map information stored on a SD memory card (1GB SD card included with package) that is inserted behind the two AA batteries that are used to power the device (see diagrams below for more information. The device is very sturdy yet compact, easily concealed yet able to withstand the elements for wherever you go.

Disassembly Procedure

A basic Philips Head computer screwdriver. The only tool required for disassembly/reassembly.

The disassembly of the DeLorme Earthmate GPS PN-20 is a fairly simple procedure. It requires the use of only one 1/16" computer screwdriver.

1. The first step to disassemble the unit is to flip it over to the back side and remove the two screws holding the battery plate in place. This can be done by hand as the two screws have handles for ease of untwisting the screws.

2. The second step to disassemble the unit is to unscrew the remained eight screws holding the bottom half of the GPS unit to the top half. For this step a screwdriver is required. The top two screws (near the lanyard loop) are the smallest screws. The remaining six screws (all of equal length and size) are fastened into place with rubber washers most likely to maintain the unit's waterproof case, two screws of the remaining six are on the sides of the eight-pin connector and the remaining four screws are removed from the wells behind the now removed battery plate. *Note* The four screws behind the battery plate, while easily unfastened from the top half of the GPS unit are difficult to remove from their respective wells and thus are not shown in the corresponding diagram.

3. The third step to disassemble the unit is to remove the upper and lower halves of the GPS unit now that the screws that have held them in place have been removed. Once opened it should resemble the corresponding image shown above on this page. The innards of the Earthmate GPS PN-20 consist of a slot to insert a SD card for topography data and other various maps, several wires connecting to both the battery power supply and the eight-pin connector. The motherboard itself contains various chips and circuits for processing data, displaying the LCD screen, and operating the button user interface on the front of the GPS unit.

Screws fastening the two halves of the GPS unit together. Two are placed on the sides of the eight-pin connector and the other four are placed underneath the battery plate.

Label 3

Screws (eighth inch in length)

Two

Screws fastening the two halves of the GPS unit together. These screws are placed near the top of the unit (near the lanyard loop)

Label 4

Screws with rotating handles (half inch in length)

Two

Screws that fasten the battery plate in place. Handles are attached to the screws so that a tool is not required to remove them.

Label 5

Battery plate

One

Plate that covers the battery power supply, also allows access to SD memory card slot

Assembly

Assembly of the DeLorme Earthmate GPS PN-20 follows the steps of disassembly except in reverse. The same tool, a simple Philip's head screwdriver, is all that is required. If the disassembly instructions were followed properly there should be difficulty reassembling the GPS unit.

After Assembly

After reassembling the GPS unit, insert two charged AA batteries and press the power symbol button on the front of the device. If the LCD screen powers up with access to all its functions, the device is ready for operation.